Chapter 15: Intro to Immune Response & Inflammation

The body relies on integrated defense mechanisms—barrier, cellular, inflammatory, and immune responses—to maintain homeostasis against foreign pathogens, injury, and nonself-cells. The barrier defenses include the physical protection of the skin and mucous membranes, the chemical activity of gastric acid, and the ability to distinguish self from nonself via the Major Histocompatibility Complex (MHC) and its identifying histocompatibility antigens (HLAs). If these defenses are breached, the mononuclear phagocyte system (MPS) activates, utilizing leukocytes such as myelocytes (like neutrophils and macrophages) which employ phagocytosis to engulf and destroy foreign material. The rapid, non-specific inflammatory response is triggered by cell injury, causing the activation of Hageman factor (Factor XII). Hageman factor initiates the kinin system, releasing bradykinin, while cell injury also releases histamine. These chemicals promote vasodilation and increased capillary permeability, resulting in the four cardinal signs of inflammation: calor (heat), tumor (swelling), rubor (redness), and dolor (pain). Further activation involves the release of arachidonic acid, which creates autocoids (such as prostaglandins and leukotrienes) that modulate the response, and the release of pyrogens from activated neutrophils, inducing fever. More specific protection is provided by the immune response, mediated by lymphocytes. T cells provide cell-mediated immunity and mature in the thymus gland. T cells include effector or cytotoxic T cells that directly destroy nonself-cells using cytokines, helper T cells (CD4) that stimulate the overall immune reaction, and suppressor T cells (CD8) that dampen it. B cells facilitate humoral immunity by recognizing specific antigens. Upon activation, B cells form plasma cells and memory cells that produce antibodies (immunoglobulins), which circulate and react with the specific antigen to form an antigen–antibody complex. This complex activates the complement proteins, a cascade that leads to the destruction (lysis) of the antigen or precipitates an aggressive inflammatory reaction. Communication within the immune system is achieved through mediators like interferons (which block viral replication) and interleukins (which communicate among leukocytes). Disruptions in this system can lead to serious conditions, including the development of neoplasms (when mutant cells evade detection), autoimmune disease (when the body reacts against self-antigens), or transplant rejection (reaction to foreign HLAs).